I was wondering why MaXius would start an oil thread!!
Welcome Maximus!
Might help us if you give us your car specs...
I think Audi usually specify a 5W30 synthetic, but I could be corrected on this...
Generally you pay for what you get with oils.
There are three oil Groupings which are used for engine oils:
Group III - mineral oils made from refined crude oil. Can include chemical additives for stablisation, such as in "Multi-Grade" oil. Generally don't get a better viscosity grade than 20W40.
TO BE AVOIDED for any decent modern engine, especially if it say's "Multigrade" which is code for a bunch of chemical stablisers which are worn out in about 3000km.
Group IV - "synthetic" oils. Basically these oils are ultra refined crude oil. They go through a process whereby undesirable hydrocarbon chains are filtered out, leaving desirable ones behind to make the oil very stable though the temperatures encountered inside an engine.
Castrol won a court case against Mobil to call Group IV oils synthetic due to the processes used to refine the oil basically tailoring the chemical stucture. Mobil didn't like it as it cheapened their achievements in Group V oils.
Group V - Synthetic oils. These oils are manufactured in a laboratory process which creates the very hydrocarbon chains desired for a nice stable oil with absolutely no contaminants or undesirable qualities. Very high temperature stabilisation and logevity can be achieved with Group V oils
Due to the manufacturing required for Group V oils, they tend to be very expensive in comparison to Group IV synthetics.
The most famous oil made to the Group V standard is/was the original Mobil 1.
Mobil 1 is one of the best if not
the best automotive engine oils around for the average consumer. But you need to be careful, as I believe Mobil have given in to the Castrol bandwagon and are now producing a Group IV version of Mobil 1, in addition to the original Group V. Not that there's anything wrong with Group IV, they are truly excellent oils and I use Nulon 5W40 in my own car, just don't get ripped off by the "Mobil 1" name: I think they're calling them Silver (Group IV) and Gold (Group V) or something similar.
Group IV's are excellent oils and the majority of "100% synthetic" oils on the market are Group IV. Group V is better, but you do pay for it.
As for viscosity: Viscosity is basically how thin an oil is, or how well it runs or lubricates. Basically, the further apart the two numbers are on a viscosity index, the more stable the oil is over temperature.
The first number is cold viscosity, and you pretty much want this as low as possible.
The second number is hot viscosity (measured at about 300 degrees IIRC) and you pretty much want this as high as possible.
So a 5W50 fully synthetic oil is an extremely high quality and stable oil.
Having said that, many new manufacturers are specifying quite low numbers for hot viscosity, such as 5W30. I'm not sure why.
